The present invention relates to a liquid crystal display apparatus and its displaying method to which a color field sequential driving method has been applied. More particularly, it relates to a liquid crystal display apparatus using the apparatus and method, such as a wearable display or a projection type display.
At present, as the methods of displaying a color image in a liquid crystal display, the following two methods can mainly be mentioned. One is a three-primary-colors color filter method, and the other is the color field sequential driving method (which is also referred to as a color frame sequential driving method).
The color filter method is as follows: One pixel is divided into three subpixels, and then the three-primary-colors color filter is located in each of the subpixels, and finally the luminance relationship among the respective colors is adjusted, thereby making it possible to implement the color display in the liquid crystal display. This method is the most common of the color display methods used at present. Meanwhile, the color field sequential driving method is as follows: Monochromatic images corresponding to the respective three primary colors are displayed in sequence in time-division at high-speed, thereby taking advantage of an afterimage effect of the eyes so as to cause the observer to visually recognize the image as a color image.
The color filter method requires that one pixel should include three subpixels in order to perform the color display. In contrast to this, the color field sequential driving method allows the color display to be performed with only one subpixel (Hereinafter, in the present specification, one subpixel in the color field sequential driving method is also represented as one pixel). Accordingly, in the color field sequential driving method, it is possible to reduce the number of the pixels down to one-third with the resolution maintained that is the same as the resolution in the color filter method. This condition makes it possible to reduce the driver circuit down to one-third, thereby allowing the power to be saved. Also, in aiming to downsize the display, for the above-described reason, the color field sequential driving method is more advantageous than the color filter method.
Moreover, in the color field sequential driving method, there is no need of using the color filter that absorbs light of unnecessary wavelength and permits light of necessary wavelength alone to pass through. Accordingly, the use of monochromatic light as the backlight makes it possible to obtain a light-utilization ratio that is even higher as compared with the case of the color filter method. Namely, there also exists an advantage that, in comparison with the color filter method, it becomes possible to exceedingly reduce the power consumption needed to achieve the same luminance.
Consequently, the color field sequential driving method having the above-described advantages is particularly important in a small-sized portable type color display required to operate with a low power consumption, such as the wearable display that is expected to become a next-generation portable type color display.
Incidentally as a literature concerning the above-described technologies, there exists Society for Information Display (SID) (99, pp. 1098-1101 N. Ogawa et al. Field-Sequential-Color LCD Using Switched Organic EL Backlighting).
FIGS. 1A to 1D illustrate data such as signal waveforms for explaining the prior arts in the color field sequential driving method.
FIGS. 1A to 1C are signal waveform diagrams for illustrating the following, respectively: FIG. 1A: time variations in driving voltages to a liquid crystal pixel (cell), FIG. 1B: time variations in driving voltages in the case where a direct voltage component is superimposed on the driving voltages to the liquid crystal pixel, FIG. 1C: time variations in luminances of the liquid crystal pixel in the case where the driving voltages in FIG. 1B are applied to the liquid crystal pixel. FIG. 1D illustrates an applied voltage-luminance characteristic in the liquid crystal pixel.
Usually, when displaying an image in a liquid crystal display, an alternating voltage as illustrated in FIG. 1A is applied to an electrode of a liquid crystal pixel, thereby driving the liquid crystal pixel. In an example in FIG. 1A, driving voltages VR, VG, and VB, which cause colors of red (R), green (G), and blue (B) to be displayed respectively in this sequence during one frame time-period 102, are applied to each liquid crystal pixel. Each of the driving voltages VR, VG, and VB is applied during a subframe time-period 103. Incidentally, although the polarity of each of the driving voltages VR, VG, and VB is inverted between adjacent frames, the sequence of the colors remains the same in each frame.
However, when the driving is executed using the alternating signal in a transistor circuit in a liquid crystal pixel included in an actual active matrix type liquid crystal display, in, for example, the liquid crystal cell, there occurs a capacitive coupling attributed to a signal electrode and the pixel electrode. This capacitive coupling superimposes a direct voltage component VDC on the driving voltages VR, VG, and VB. FIG. 1B illustrates, as the concrete example, the case where the direct voltage component VDC (in the case in FIG. 1B, VDC greater than 0) is superimposed on the driving voltages. Additionally, for your information, the case illustrated in FIG. 1A can be considered as an ideal case where VDC=0. In the example illustrated in FIG. 1B, the direct voltage component by the amount of VDC is added to the driving voltage waveforms illustrated in FIG. 1A. Namely, the driving voltage waveforms in FIG. 1B are the same as those in FIG. 1A, but are shifted onto the plus side by the amount of VDC. Consequently, even when the same color is displayed in the same liquid crystal cell during a time-period of a certain plurality of frames, the absolute value of the driving voltage for displaying the same color turns out to become different between the adjacent frames between which the polarity of the driving voltage differs (in the case illustrated in FIG. 1B, the driving voltage differs by the amount of 2VDC). Eventually, towards the pixel having one and the same color, the absolute value of the driving voltage differs between the adjacent frames. This means that the luminance corresponding to the driving voltage differs between the adjacent frames as illustrated in the characteristic diagram in FIG. 1D. FIG. 1C illustrates, introducing the difference in each luminance, a time variation in each luminance corresponding to each driving voltage waveform in FIG. 1B. As is obvious from FIG. 1C, even when the same color is displayed continuously in the same liquid crystal cell, the next frame turns out to become distinguishable because the luminance differs between the adjacent frames. As a consequence of this, the two frames become one period, thus causing flicker (which, here, means a slight amount of blinking of the luminance) to occur. Here, the flicker is synchronized with a frequency that is equal to one-half of the frame frequency.
In order to prevent this flicker, in an ordinary liquid crystal display, the following driving is performed: Towards the pixel having one and the same color, the polarity of the driving voltage is inverted for each column and/or for each row.
Applying the above-described driving method, however, results in inverting polarities of the driving voltages to each other, the driving voltages occurring in two pixels existing in adjacent columns and/or adjacent rows. This causes a disturbance in an electric field to occur in proximity to the boundary of the pixels. As a result, there occurs a liquid crystal orientation failure in proximity to the boundary of the pixels. The region where the liquid crystal orientation failure has happened is recognized as a display failure. Concealing, with a light-shielding frame, the region where the liquid crystal orientation failure has happened prevents the display failure from being visually recognized, but results in decreasing an aperture ratio greatly. Furthermore, in the case where the pixel pitch is made narrower in order to implement a high-resolution and to downsize the display, a percentage at which the display failure region occupies the entire display region is increased. This makes it inevitable to bring about a serious problem of decreasing the aperture ratio exceedingly. Accordingly, in order to aim to implement the high-resolution and to downsize the display, it is after all required to apply, towards the pixel having one and the same color within the one frame time-period, the driving voltage the polarities of which are identical to each other in both the adjacent columns and the adjacent rows (This driving method is referred to as a frame inversion driving). In this frame inversion driving, however, the problem of the flicker resulting from the above-described direct voltage component still remains without being solved. This requires that this problem should be solved by a method other than the above-described method.
Accordingly, it is an object of the present invention to provide a liquid crystal display apparatus and its displaying method, the apparatus and the method preventing the flicker that occurs when the frame inversion driving is performed in the color field sequential driving, and being adaptable to the implementation of the high-resolution and the downsizing of the display.
According to one aspect of the present invention, there is provided a liquid crystal display apparatus, including:
a display unit including a plurality of pixels, and
a driving unit for sequentially applying driving voltages for monochromatic images to each of the plurality of pixels included in the display unit so as to cause each of the pixels to sequentially display the monochromatic images, the driving unit employing, as one unit, a time-sequential arrangement of the driving voltages for the 2s (s is an integer equal to or larger than 2) monochromatic images that include three primary colors of red, blue, and green, and sequentially applying the one unit of arrangement of the driving voltages periodically to each of the pixels included in the display unit so as to cause each of the pixels to sequentially display the monochromatic images arranged in accordance with the arrangement, wherein a color of the monochromatic image is any one of the three primary colors of red, blue, and green, each of the pixels included in the display unit being caused to display the monochromatic image at one point in time.
Accordingly, a polarity of the driving voltage in the monochromatic image having one and the same color always remains one and the same polarity. This makes it possible to exceedingly decrease a difference between absolute values of the driving voltage caused by the polarity inversion of the driving voltage. As a result, it becomes possible to provide a high picture-quality liquid crystal display apparatus exhibiting no flicker.
Also, in addition to the above-described configuration, the following configuration is provided: A polarity of a driving voltage applied to a pixel is controlled arbitrarily for each monochromatic image, thereby making polarities of driving voltages identical to each other, the driving voltages being applied to at least two monochromatic images having one and specified color. This allows conditions of the driving voltages to be classified depending on the cases, thereby making it possible to eliminate a direct voltage component that brings about a degradation in the picture-quality. As a result, it becomes possible to provide a high picture-quality liquid crystal display apparatus.
According to another aspect of the present invention, there is provided a liquid crystal display apparatus, including:
a display unit including a plurality of pixels, and
a driving unit for sequentially applying driving voltages for monochromatic images to each of the plurality of pixels included in the display unit so as to cause each of the pixels to sequentially display the monochromatic images, the driving unit employing, as one unit, a time-sequential arrangement of the driving voltages for the 2s (s is an integer equal to or larger than 2) monochromatic images that include three primary colors of red, blue, and green, and sequentially applying the one unit of arrangement of the driving voltages periodically to each of the pixels included in the display unit so as to cause each of the pixels to sequentially display the monochromatic images arranged in accordance with the arrangement, wherein the driving voltage for the monochromatic image is any one of the driving voltages for red, blue, and green, and the 1st driving voltage, the driving voltage being applied at one point in time to each of the pixels included in the display unit.
Accordingly, a polarity of the driving voltage in the monochromatic image always remains one and the same polarity. This makes it possible to exceedingly decrease a difference between absolute values of the driving voltage caused by the polarity inversion of the driving voltage. As a result, it becomes possible to provide a high picture-quality liquid crystal display apparatus exhibiting no flicker.
Furthermore, in addition to the above-described configuration, the following configuration is provided: In a time-period during which one monochromatic image included within the one periodic arrangement is displayed, each of the pixels included in the display unit is not irradiated with light or an observer is prevented from recognizing the light visually, and in the time-period, the driving voltage applied to each of the pixels is set as the 1st driving voltage (correcting voltage). This allows the driving voltage to be set as the correcting voltage, the driving voltage existing in the time-period during which the observer is prevented from recognizing the light visually, and also makes it possible to eliminate the direct voltage component for each periodic arrangement. As a result, it becomes possible to provide the liquid crystal display apparatus exhibiting no flicker and with the high picture-quality.
Also, according to another view point of the present invention, there is provided a liquid crystal display apparatus including a display unit and a driving unit, wherein a color of a monochromatic image that the driving unit displays is any one of the three primary colors, and one frame includes 2s (s is an integer equal to or larger than 2) subframes. Accordingly, a polarity of the driving voltage in the monochromatic image having one and the same color always remains one and the same polarity. This makes it possible to exceedingly decrease a difference between absolute values of the driving voltage caused by the polarity inversion of the driving voltage. As a result, it becomes possible to provide a high picture-quality liquid crystal display apparatus exhibiting no flicker.